1. Field of the Invention
The invention relates to a tunable laser source and, more particularly, to a tunable laser source that utilizes a wavelength selection device to select a desired wavelength for the tunable laser source.
2. Description of the Related Art
Recently, a variety of techniques have been applied to tune diode lasers. For instance, U.S. Pat. No. 5,319,668 discloses a tunable diode laser with a diffraction grating and a rotatable mirror for wavelength selection. Referring to FIG. 1, such tunable diode laser 100 includes a solid state diode laser 102, a collimating lens 104, a grating 106 and a rotatable mirror 108. The light emitted from the solid state diode laser 102 passes through the collimating lens 104, and the collimated light illuminates the grating 106 at a grazing angle such that a first-order diffraction beam 110 emerges from the grating 106. The first-order diffraction beam 110 then strikes the mirror 108 to provide a return beam to the solid state diode laser 102 and to cause oscillation at a wavelength determined by the diffraction angle of the first-order diffraction beam 110 perpendicular to the mirror 108. Hence, the output wavelength of the tunable diode laser 100 can be tuned by adjusting the inclination of the mirror 108.
However, such configuration often needs a complex rotation mechanism, and the dimensions from the pivot of the rotation mechanism to the collimating lens 104 and the mirror 108 require high precision, which is difficult to ensure under temperature variations. Also, because the mirror must be wide enough to intercept all diffracted beams, it is difficult to minimize the size of the rotation mechanism and thus difficult to increase its modulation speed, which is needed by broadband wavelength tuning.